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    <script>
        var num = 1;
        // num = num + 1;
        ++num;
        console.log(num);
        // 前置运算符， ++ 写在变量得到前面
        var age = 10;
        console.log(++age + 10);    //21
        console.log(age);   //11
        // 先加1，再返回。
    </script>
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